In many military applications, there is a strong need to make aircraft, vehicles and other objects, including their component parts, as invisible to radar as possible. A number of techniques for accomplishing this purpose are well known. One such technique is to coat metallic parts with a ceramic radar-absorbing material.
Conventionally, the ceramic RAM is suspended in particulate form in a wet slurry which is sprayed onto the substrate of the part to be processed. Although this process is easy to use and is performable with readily available equipment, and has proven generally suitable for its intended purpose, it possesses inherent deficiencies which detract from its overall effectiveness and desirability. Specifically, the spray process has several disadvantages: for one, coatings of small, complex parts or parts with small internal diameters are difficult to obtain consistently; secondly, the spray process does not lend itself well to automation because variables such as coating thickness are difficult to control; and thirdly, a sprayed coating sometimes has difficulty adhering to the part with the result that electromagnetic performance is degraded.
Furthermore, because the slurry is a mixture of heavy and light particulates, it is important to maintain the slurry in a homogenous consistency. This can be done by maintaining the slurry in an agitated and/or flowing state, which keeps the heavier particles in suspension.
Other prior art methods include the following:
Nishio et al. U.S. Pat. No. 5,091,222 describes a method of ceramic coating in which the workpiece is dipped into a ceramic solution;
Van 'T Veen et al. U.S. Pat. No. 5,089,299 shows apparatus for applying a micropore coating to a ceramic substrate, in which the workpiece is moved with respect to the ceramic suspension. This is undesirable because movement of the part can disrupt the uncured coating.
Reed et al. U.S. Pat. No. 4,208,454 shows a coating process in which an alumina slurry is forced though a workpiece by a vacuum.
In view of the shortcomings of the prior art, it is desirable to provide a process which will uniformly coat parts regardless of their size or complexity, and will not be subject to the inherent inconsistencies arising from variations in spray patterns and from non-homogenity of the slurry. In this regard, although the prior art has recognized to a limited extent the nature of this problem, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy.